In literature there are lot of types of pumps and pump impellers for this purpose described, all however having certain disadvantages. Above all this concerns problems with clogging and low efficiency.
Sewage water contains a lot of different types of pollutants, the amount and structure of which depend on the season and type of area from which the water emanates. In cities plastic material, hygiene articles, textile, etc, are common, while industrial areas may produce wearing particles. Experience shows that the worst problems are rags and the like which stick to the leading edges of the vanes and become wound around the impeller hub. Such incidents cause frequent service intervals and a reduced efficiency.
In agriculture and pulp industry, different kinds of special pumps are used, which should manage straw, grass, leaves and other types of organic material. For this purpose the leading edges of the vanes are swept backwards in order to cause the pollutants to be fed outwards to the periphery instead of getting stuck to the edges. Different types of disintegration means are often used for cutting the material and making the flow more easy. Examples are shown in SE-435 952, SE-375 831 and U.S. Pat. No. 4,347,035.
As pollutants in sewage water are of other types more difficult to master, and as the operation times for sewage water pumps normally are much longer, the above mentioned special pumps do not fullfil the requirements when pumping sewage water, neither from a reliability nor from an efficiency point of view.
A sewage water pump quite often operates up to 12 hours a day which means that the energy consumption depends a lot on the total efficiency of the pump.
Tests have proven that it is possible to improve efficiency by up to 50 % for a sewage pump according to the invention as compared with known sewage pumps. As the life cycle cost for an electrically driven pump normally is totally dominated by the energy cost (about 80% of the life cycle cost), it is evident that such a dramatic increase will be extremely important.
In literature the designs of the pump impellers are described very generally, especially as regards to the sweep of the leading edges. An unambigous definition of said sweep does not exist.
Tests have shown that the design of the sweep angle distribution on the leading edges is very important in order to obtain the necessary self cleaning ability of the pump impeller. The nature of the pollutants also calls for different sweep angles in order to provide a good function.
Literature does not give any information about what is needed in order to obtain a gliding, transport, of pollutants outwards in a radial direction along the leading edges of the vanes What is mentioned is in general that the edges shall be obtuse-angled, swept backwards etc. See SE-435 952.
When smaller pollutantans such as grass and other organic material are pumped, relatively small angles may be sufficient in order to obtain the radial transport and also to disintigrate the pollutants in the slot between pump impeller and the surrounding housing. In practice disintigration is obtained by the particles being cut through contact with the impeller and the housing when the former rotates having a periphery velocity of 10 to 25 m/s. This cutting process is improved by the surfaces being provided with cutting devices, slots or the like.
Different sorts of notches and cutting means are described in SE-435 952 and SE-375 831. They all have in common that the vane is located behind a shoulder This means a considerable loss of efficiency as compared with an even contour which is used in high efficiency pumps for clean water.
In SE-435 952, an embodiment is shown where an axial aperture is located behind a shoulder. The theory is that pollutants shall be fed outwards to said aperture by the vanes having leading edges strongly swept backwards. This embodiment described very generally, is however not suitable to pump heavy pollutants contained in sewage water.
In SE-375 831, a solution is described using the opposite principle that pollutants are transported towards the centre, away from the slot. This fact, in combination with the previously mentioned shoulder, makes feeding into the slot impossible.
As previously mentioned, it is a condtion that the leading edges of the vanes are swept strongly backwards in order to make a transport of the pollutants outwards and into the slot at the periphery possible. If this is not obtained, serious shut downs will occur very soon. Pump impellers of this type are described in SE-9704222-0 and SE-9704223-8. When the pollutants slide outwards and reach the slot between the vane and the pump housing wall, there is however a risk that they will stick to the periphery of the leading edge and clog within the slot.
In DE-614 426 there is shown a device meant to solve such problems, without the need for the previously mentioned shoulder. The pump is a centrifugal pump having a very sharp linking from the axial inlet to the radial part of the flow channel. The periphery of the leading edge is located downstream of said linking in the radial part of the channel.
A device is further mentioned which has a solid notch in front of the leading edge with a decreasing height up to a cutting knife, followed by a spiral formed groove with a triangular cross section and sharp corners and which widens towards the periphery. In addition it is stated that the basic principle for this type of solution is that the replacable cutting means shall disintegrate the pollutants. If this should fail, for. instance if the cutting means is blunt, the consequence will be that the decreasing height of the notch will compress the pollutants to clogg where the area has its minimum, i.e. within the area of said cutting means.
The above mentioned patent thus describes a solution which, under certain conditions, may obtain a self cleaning ability, but which has got important disadvantages concerning efficiency, wear resistance and life. In addition, there are no details given about the very important conditions regarding the leading edges of the vanes and thus it has no meaning to try to apply this described device when pumping sewage water.